The present invention relates to fine spinel-type ferrimagnetic particles containing Fe, Co and Ni, and a process for producing the same. More particularly, the present invention relates to fine spinel-type ferrimagnetic particles containing Fe, Co and Ni, which can exhibit a high coercive force in spite of fine particles, and a process for producing the fine spinel-type ferrimagnetic particles in an aqueous solution at a temperature of not more than 101xc2x0 C.
Conventionally, as magnetic materials used in recording media such as data-filing tapes, there has been proposed spinel-type ferrite in the form of an iron-containing composite oxide. The spinel-type ferrite has various compositions represented by the formula: MFe2O4, wherein M is Fe(II), Co, Ni, Mn, Al, Cu, Zn or the like. The spinel-type ferrite includes, for example, Co ferrite. The Co ferrite has a large crystal magnetic anisotropy constant and, therefore, is unlikely to form superparamagnetic particles in spite of finer particles. As a result, the Co ferrite can maintain good magnetic properties, so that it has been expected to apply the Co ferrite to high-density magnetic recording materials. The Co ferrite is formed into a thin film by a spin-spray ferrite plating method.
As magnetic materials having more excellent magnetic properties, there are known Co-Ni ferrite thin films (Cho and other two persons, xe2x80x9cEffects of Addition of Ni to Co Ferrite Thin Filmxe2x80x9d, xe2x80x9cPowder and Powder Metallurgyxe2x80x9d, Feb. 25, 2000, Vol. 47, No. 2, pp. 171-174).
The above Co-Ni-based spinel ferrite is a ferrimagnetic material, and is used in the form of a bulky mass or a thin film. However, since the Co-Ni-based spinel ferrite has a coercive force as low as less than 239 kA/m (3,000 Oe), it may be difficult to produce recording media having high-recording density, by using such a magnetic material. Namely, although the Co-Ni-based spinel ferrite is applicable to tape-shaped recording media, there arises such a problem that the recording media are no longer improved in performance thereof since the coercive force of the Co-Ni-based spinel ferrite cannot be further enhanced.
On the other hand, magnetoplumbite-type ferrite (M-type ferrite) as magnetic material applied to recording media, e.g., Ba ferrite, has also been studied for reducing the particle size thereof. However, Ba ferrite magnetic particles having an average particle diameter of 30 nm to 40 nm, exhibit a coercive force as low as less than 239 kA/m (3,000 Oe).
That is, since the conventional magnetic materials have failed to be further improved in coercive force, it may be difficult to obtain recording media having a higher recording density by using such magnetic materials.
As a result of the present inventors"" earnest studies for solving the above problems, it has been found that by mixing aqueous solutions containing Fe3+, Co2+ and Ni2+, respectively, which are prepared by dissolving water-soluble salts of iron, cobalt and nickel in water, at a predetermined molar ratio; while stirring the resultant mixed solution, adding thereto an aqueous alkali solution in an amount of not less than one equivalent based on whole metal ions contained in the mixed solution such that the concentration of OH groups contained in a solution obtained after adding the aqueous alkali solution is not more than 3 mol/liter; and heating the resultant precipitated slurry under stirring at a temperature of 80 to about 101xc2x0.,
the thus obtained fine spinel-type ferrimagnetic particles are a magnetic material capable of exhibiting not only a high coercive force in spite of fine particles, but also such very excellent magnetic properties that a rotational hysteresis integration value thereof relating to rotational hysteresis loss and magnetization mechanism are close to Fanning Model. The present invention has been attained on the basis of this finding.
An object of the present invention is to provide fine spinel-type ferrimagnetic particles useful as a high-density magnetic recording material capable of exhibiting not only a high coercive force in spite of fine particles but also a rotational hysteresis integration value relating to rotational hysteresis loss and magnetization mechanism which is close to Fanning Model.
Another object of the present invention is to provide a process for producing fine spinel-type ferrimagnetic particles of a single-phase structure, which are capable of exhibiting a high coercive force in spite of fine particles, in an aqueous solution at a temperature of 80 to about 101xc2x0 C. using a simple apparatus.
To accomplish with the aims, in a first aspect of the present invention, there are provided fine spinel-type ferrimagnetic particles having a composition represented by the formula (I):
(CoO)x(NiO)y.n/2Fe2O3xe2x80x83xe2x80x83(I) 
wherein 2.0 less than n=Fe/(Co+Ni) less than 3.0, 0.4xe2x89xa6xxe2x89xa60.9 and 0.1xe2x89xa6yxe2x89xa60.6.
In a second aspect of the present invention, there is provided a process for producing fine spinel-type ferrimagnetic particles, comprising:
(1) mixing aqueous solutions containing Fe3+, Co2+ and Ni2+ with each other, which are prepared by dissolving each of water-soluble salts of iron, cobalt and nickel in water, at such ratio of an Fe mole to the sum of a Co mole and a Ni mole being from more than 2.0 to less than 3.0 and the Co mole to the Ni mole being 0.4 to 0.9/0.6 to 0.1,
(2) while stirring the resultant mixed solution, adding thereto an aqueous alkali solution in a range of from not less than one equivalent based on whole metal ion contained in the mixed solution to an amount such that the concentration of OH groups contained in a solution obtained after adding the aqueous alkali solution is not more than 3 mol/liter; and
(3) heating the resultant precipitated slurry at a temperature of 80 to 101xc2x0 C. while stirring.
In a third aspect the present invention, there are provided Fine spinel-type ferrimagnetic particles having a composition represented by the formula (I):
(CoO)x(NiO)y.n/2Fe2O3xe2x80x83xe2x80x83(I) 
wherein 2.0 less than n=Fe/(Co+Ni) less than 3.0, 0.4xe2x89xa6xxe2x89xa60.9 and 0.1xe2x89xa6yxe2x89xa60.6 produced by a process comprising:
(1) mixing aqueous solutions containing Fe3+, Co2+ and Ni2+ with each other, which are prepared by dissolving each of water-soluble salts of iron, cobalt and nickel in water, at such ratio of an Fe mole to the sum of a Co mole and a Ni mole being from more than 2.0 to less than 3.0 and the Co mole to the Ni mole being 0.4 to 0.9/0.6 to 0.1,
(2) while stirring the resultant mixed solution, adding thereto an aqueous alkali solution in a range of from not less than one equivalent based on whole metal ion contained in the mixed solution to an amount such that the concentration of OH groups contained in a solution obtained after adding the aqueous alkali solution is not more than 3 mol/liter; and
(3) heating the resultant precipitated slurry at a temperature of 80 to 101xc2x0 C. while stirring.
In a fourth aspect the present invention, there is provided a magnetic sheet comprising a binder resin and fine spinel-type ferrimagnetic particles having a composition represented by the formula (I):
(CoO)x(NiO)y.n/2Fe2O3xe2x80x83xe2x80x83(I) 
wherein 2.0 less than n=Fe/(Co+Ni) less than 3.0, 0.4xe2x89xa6xxe2x89xa60.9 and 0.1xe2x89xa6yxe2x89xa60.6.
In a fifth aspect the present invention, there are provided fine spinel-type ferrimagnetic particles comprising single-phase structure, comprising iron, cobalt and nickel, the ratio of an iron mole to the sum of a cobalt mole and a nickel mole being from more than 2.0 to less than 3.0, and
having a particle diameter of 10 to 50 nm, a saturation magnetization "sgr"s of 50.3xc3x9710xe2x88x926 to 88.0xc3x9710xe2x88x926 Wb.m/kg and a coercive force HcJ of 239 to 637 kA/m.